Metal Organic Chemical Vapor Deposition (MOCVD) and Organometallic Vapor Phase Epitaxy (OMVPE) are widely used in forming semiconductor epitaxial layers on semiconductor substrates. In particular, they are utilized to grow epitaxial semiconductors containing compound semiconductors such as Gallium Arsenide (GaAs), Indium Gallium Arsenide (InGaAs), and Indium Phosphide (InP). These compound semiconductors have advantageous properties when compared to conventional silicon semiconductors and they are primarily used for semiconductor devices such as semiconductor lasers, high frequency microwave monolithic integrated circuits, and electro-optical devices.
In a typical MOCVD process, semiconductor substrates are first placed in a reaction chamber. The substrates are then heated under a hydrogen flow. Gases, including some containing vapors of metal organic compounds, are then introduced into the reaction chamber. As a result of chemical reaction at the surfaces of the heated substrates, thin layers of semiconductors are formed on the surfaces of the substrates. By continuously providing the gases and heating the substrates, the thicknesses of the semiconductor layers continuously increase. When the semiconductor layers reach desired thicknesses, the process is terminated by discontinuing supply of the reactant gases.
In the MOCVD process, the growth rate and the composition of the epitaxially grown layers depend on the delivery rates of the metal organic compounds. To form semiconductor epitaxial layers with desired composition and thickness, the delivery rate of each metal organic compound needs to be precisely controlled. Such precise control of the delivery rate is critical for fabricating semiconductor devices like semiconductor lasers which may contain epitaxial layers having thicknesses on the order of less than a hundred Angstroms.
Trimethylindium (TMI) is a widely used metal organic compound for providing the indium (In) used in forming epitaxial semiconductor layers such as Indium Gallium Arsenide (InGaAs). TMI is a solid metal organic compound at room temperature. Solid TMI sublimes (i.e., turns directly into vapor) at a rate proportional to its surface area and temperature. To introduce TMI into an MOCVD reaction chamber, TMI vapor is first generated by heating solid TMI. The vapor is then incorporated into a carrier gas and is subsequently delivered to the reaction chamber. The carrier gas is usually hydrogen (H.sub.2) or an inert gas such as argon (Ar) or helium (He). The carrier gas functions as a vehicle that transports the vapor of a metal organic compound into the MOCVD reaction chamber.
High surface area TMI is usually employed in producing TMI vapor. However, during extended use, TMI recrystallizes and turns into large crystalline lumps thereby effectively reducing the surface area. As a result, even if other conditions remain unchanged during the process, the TMI delivery rate gradually decreases, thereby undesirably affecting the composition and growth rate of the epitaxially formed semiconductor layers.
Similar problems are encountered in Chemical Beam Epitaxy (CBE) or Metal Organic Molecular Beam Epitaxy (MOMBE) or any epitaxial technique in which the Group III source is supplied as a solid organometallic compound. The methodologies involved in these techniques are described by Stringfellow in Organometallic Vapor Phase Epitaxy, Academic Press, Inc., 1989, and by Tsang in "From Chemical Vapor Epitaxy to Chemical to Chemical Beam Epitaxy", Journal of Crystal Growth, 95(1989), pp.121-131. In addition, transport problems can be encountered in other chemical vapor deposition processes that use solid chemical sources such as deposition of aluminum from trimethylamine alane, deposition of copper from copper (II) hexafluoroacetylacetonate, or deposition of titanium nitride from tetrakis(dimethylamido) titanium (IV).
To overcome this problem, methods and apparatuses are disclosed in U.S. Pat. Nos. 4,916,828 and 5,019,423 to produce a stable delivery of solid metal organic compounds by saturating the carrier gas with metal organic vapor. When using this approach, the delivery rate for TMI remains stable over a long period of time. However, gas saturated with TMI vapor can easily condense in the tubing that leads the gas to the reaction chamber. Such condensation produces blockage in the tubing and alters the delivery rate of TMI.
It is therefore an object of this invention to generate and deliver at a stable rate a gas that contains a vapor produced by subliming a solid.